The invention relates to a valve drive for an internal combustion engine and, more particularly, to a valve drive for an internal combustion engine having a camshaft mounted in bearings and with at least one axially displaceable cam. The cam, with interposition of a transfer element, actuates at least one gas-exchange valve.
German Patent Document DE OS 20 37 705 describes a known valve drive with a hydraulically axially displaceable camshaft, on which fixed cams with two different elevation curves are mounted. When the oil pressure is switched on, gas-exchange valves with a larger valve lift are actuated in one end position of the camshaft, while when the oil pressure is shut off the camshaft, displaced into a second end position under spring tension, actuates the valves with a lesser valve lift.
A disadvantage of the known valve drive in this regard is the simultaneous axial displacement of all of the cams. As a result of the angular offset of the cams with respect to one another, in a multicylinder internal combustion engine, there cannot be any position of the camshaft in which all of the cams simultaneously travel on their base circles as would be required for axial displacement. In addition, in internal combustion engines with only one camshaft per row of cylinders, the cams associated with the exhaust valves are displaced as well.
A valve drive is known from German Patent Document DE-29 50 656 A1 having cams that are displaceable, as well as axially and helically rotatable, on the camshaft. The cams are held positively on the camshaft by spiral threads. Axially displaceable shafts run parallel to the camshaft. The shafts engage the cams externally in sliding fashion by dogs. The shafts rotate the cams relative to the camshaft to adjust optimum control times. The cams actuate gas-exchange valves with interposition of transfer elements designed as valve lifters. This arrangement requires considerable space because of the axially displaceable shafts. Further, the arrangement is heavy and is prone to wear due to the sliding contact between the rotating cams and dogs. In addition, a spiral toothing on the camshaft is costly to manufacture.
There is therefore needed a valve drive that overcomes these disadvantages.
These needs are met according to the present invention which provides a valve drive for an internal combustion engine having a camshaft mounted in bearings and with at least one axially displaceable cam. The cam, with interposition of a transfer element, actuates at least one gas-exchange valve. The cam is designed as a non-rotatable sliding cam that is displaceable relative to a camshaft and axially pretensioned in a first position positively abutting a stop provided between the sliding cam and the transfer element. The valve drive of the present invention has the advantage that the gas-exchange valves of all cylinders in one row of cylinders can be actuated with this valve drive. The sliding cams then rest, axially pretensioned, against the stops for as long as they rest against this stop by the portions of their ends that are delimited by the elevation curves. When the base circle of each sliding cam reaches the transfer element, the sliding cams are displaced and ride with their elevation curves on the transfer element.
This valve drive can be used, for example, for directly actuated valves, with the transfer element designed as a cup tappet provided with hydraulic valve play adjustment. In addition, the valve drive can be used in valve drives provided with levers. The transfer element is then designed as a rocker arm, valve lifter, or the like.
The sliding cams can be provided with two elevation curves, with the section abutting the stop being provided between the two elevation curves.
The various elevation curves travelling on the transfer element can have an angular offset with respect to one another, so that a phase shift results. This phase shift can be used jointly with the variation of the valve travel or instead of the latter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.